A typical six-wheel drive arrangement for a motor vehicle includes an axle having steerable wheels at the front end of the vehicle and tandem axles at the rear of the vehicle. There are different types of drive arrangements that can be used for providing drive to the tandem axles and one such arrangement that has been applied to large Class 8 semi-tractors has a torque splitter or inter-axle differential incorporated into the front axle of the tandem axle pair. The inter-axle differential is a bevel gear type that provides a 50/50 torque split between the two rear axles. Torque from the propeller shaft drives the carrier member of the inter-axle differential and outputs are through the side gears to the two rear axles. More specifically, drive from the inter-axle differential is directed to the forward axle through helical gears and to the rear axle gearing through a bevel gear. The tandem axle drive arrangement has lockout capabilities provided by the use of a sliding collar that connects the input shaft to one of the side gears of the inter-axle differential so as to force both axle pinions to rotate at the same speed. Each of the tandem axles also has a bevel gear differential so side to side wheel rotation differentiation can occur in either axle. In addition, each of the axles can be provided with a planetary gear reducer located adjacent the bevel gear differential. The planetary gear reducer can be selectively operated and used or bypassed so as to give the vehicle the capability to use a different drive ratio for starting or for low speed operation.
One problem with a tandem axle drive arrangement of the type described above is that by integrating the inter-axle differential with the usual differential of the forward axle, a large bulky unit is provided that is difficult and expensive to service and maintain. Another problem with this type of drive arrangement is that it is difficult to upgrade the inter-axle differential unit without extensive redesign of the complete forward axle for future improvements such as greater torque capacity, on demand or different torque split, multiple speeds, and use of electronics.